There is a single reason why a lens with a wide max aperture is called fast - it allows for a higher - faster - shutter speed at the same ISO, to help avoid camera shake or freeze fast moving objects . It also provides a brighter finder image for SLR cameras, and helps to focus more precisely .

Shallow DOF is only a side effect, and can be used as a photographic effect if desired, but is not the reason 'fast' lenses were being introduced originally . In general photography, and outside of the amateur enthusiast realm , DOF is merely one of many aspects that need to be controlled by the photographer, and a creative tool only in very view applications .

A f1.2 lens will always be a f1.2 lens; the single purpose of this f-number is to determine the resulting shutter speed at a certain ISO value.

This result will always be the same, no matter the focal length, format, sensor or film; ISO values are (supposedly ) absolute figures and already include sensor/film specs, and (actual, not equivalent) focal length is part of the f-number calculation .

DOF comparisons for different sensor formats can be made, like focal length comparisons, but that's not part of the technical lens specifications at all .

Also, film/sensor size based DOF calculations are a lot more complex than comparative angle of view calculations; the latter gives you the 'crop-factor', which can be applied to a certain focal length to simplify lens/format comparisons, but the crop factor (or rather FL and format size) is only one part of the DOF measurement .